The present invention relates generally to integrated circuits and specifically to termination resistor circuits.
Output driver circuits for driving cables which interconnect integrated circuits (ICs) are well-known. For example, FIG. 1 shows a well-known driver 100 fabricated using CMOS technology. NMOS transistors MN1 and MN2 form a differential pair which, in response to a differential voltage signal V2xe2x88x92V1, steers a bias current Ibias between terminating resistors 102 and 104, respectively, to produce a differential output signal between output nodes OUT_1 and OUT_2. Resistor 102 sets the minimum voltage at node OUT_1, and thus controls the voltage swing at node OUT_1. Similarly, resistor 104 sets the minimum voltage at node OUT_2, and thus controls the voltage swing at node OUT_2. The output signals on output nodes OUT_1 and OUT_2 may be used to drive a load 106 via transmission lines T1 and T2, which have a characteristic impedance ZT of between 50 ohms and 300 ohms.
To minimize signal reflections on transmission lines T1 and T2, terminating resistors 102 and 104, as well as the resistance of load 106, is chosen to match the characteristic impedance ZT of transmission lines T1 and T2. Typically, resistors 102 and 104 are passive resistive elements such as, for example, polysilicon thin film resistors. However, because of process variations inherent in the fabrication of semiconductor circuits (e.g., imprecise doping and photolithographic techniques), as well as temperature-dependent operating characteristics, such passive resistors may vary as much as 20%, which may be unacceptable for some communication applications.
For improved precision, passive resistors 102 and 104 may be replaced by active resistive elements such as, for example, NMOS transistors 202 and 204, as shown in FIG. 2. As well-known in the art, resistive transistors 202 and 204 are operated in the triode region as voltage-controlled resistances having gates to receive a control voltage VCTL. However, although more accurate than polysilicon resistors 102/104, transistors 202 and 204 may vary as much as 10% because of process and temperature variations. In addition, the p/n junctions within transistors 202 and 204 (e.g., source/well and drain/well junctions) may add significant capacitance loading to output nodes OUT_1 and OUT_2, which in turn undesirably limits circuit speed.
Accordingly, there is a need for precise termination resistors fabricated using current CMOS processes that are insensitive to temperature and process variations and which have a minimal impact upon circuit speed. In addition, for applications where the characteristic impedance of the transmission lines is not known, it would be desirable for a user to be able to change the value of the termination resistors.